The present invention relates to a visual display apparatus and, more particularly, to a visual display apparatus, e.g. a head-mounted display apparatus, which enables observation of an image at a wide field angle and high resolution.
In recent years, helmet- and goggle-type head-mounted display apparatuses have been developed for virtual reality or for the purpose of enabling the user to enjoy a wide-screen image personally.
Incidentally, a conventional head-mounted display apparatus is arranged as shown in FIG. 16 [see Japanese Patent Application Unexamined Publication (KOKAI) No. 3-188777]. An image displayed on a two-dimensional display device 51, e.g. a liquid-crystal display device, is reflected by a half-mirror 52 disposed in front of a user's eyeball at a tilt angle of 45 degrees. The reflected display image is enlarged by a concave mirror 53 disposed forward of the half-mirror 52, and the enlarged display image is viewed through the half-mirror 52. It should be noted that the two-dimensional display device 51 is disposed near the front focal point of the concave mirror 53.
Assuming that in the optical system shown in FIG. 16, WD is the distance (working distance) between the optical system and the user's eyeball, D is the diameter of the concave mirror 53, and t is the thickness of the optical system, and considering the passing area for principal rays, the following equations hold. It should be noted that D and t are approximately equal to the vertical and horizontal widths (as viewed in the figure), respectively, of the half-mirror 52, which is tilted at 45 degrees; therefore, D and t are approximately equal to each other.
D=2(t+WD)tan(.theta./2) PA1 t=D PA1 t{1-2tan(.theta./2)}=2WDtan(.theta./2) PA1 t=2WDtan(.theta./2)/{1-2tan(.theta./2)}
In view of the operability of the head-mounted display apparatus, it is necessary to set WD in the range of from 10 millimeters to 40 millimeters. In a case where WD is 20 millimeters and the field angle .theta. is 40 degrees, for example, the thickness t of the optical system becomes as large as 53 millimeters as will be understood from the above equations. Moreover, it is necessary in actual practice to ensure a passing area for subordinate rays in addition to the principal rays. Accordingly, a larger optical system is needed.
There is another problem that because a small-sized two-dimensional display device does not have a large number of pixels, the enlarged image of the display device for observation cannot have sufficiently high resolution. Under these circumstances, the present inventor examined a scheme of obtaining a wide image display area by providing a plurality of two-dimensional display devices in a side-by-side relation to each other. However, another problem is associated with this scheme. That is, a lightweight liquid-crystal display device is commonly used in a display apparatus of the type which is fitted to an observer's head as in a head-mounted display apparatus, and the liquid-crystal display device has an image display structure in which, as shown in the front view of FIG. 17, an image display area G is surrounded by a substrate portion K. Therefore, if a plurality of such liquid-crystal display devices are disposed in a side-by-side relation to each other, the substrate portions K of each pair of adjacent display devices interfere with each other, making it difficult to obtain the desired wide image display area.
Attempts have heretofore been made to widen the observation field angle by combining two or more two-dimensional display devices. However, U.S. Pat. No. 5,157,503 and German Pat. No. 4,127,924 are not such compact and lightweight apparatuses as to be fittable to an observer's head. In Japanese Patent Application Unexamined Publication (KOKAI) No. 6-308425, aberrations occur to a considerable extent because an ocular optical system is formed by using a convex lens. Moreover, aberrations change considerably when the observer's eyeball position is shifted from the proper position. Accordingly, a variety of observers having different interpupillary distances cannot stably observe the displayed image.